A decorative resin-molded article decorated by laminating a decorative sheet on the surface of a molded article is used in various applications such as automotive interior parts. The method of forming such a decorative resin-molded article includes insert molding in which a decorative sheet is first three-dimensionally formed with a vacuum forming mold, the molded sheet is inserted in an injection mold, and then a fluid state of resin is injected in the injection mold to integrate the resin with the molded sheet (for example, see Patent document 1); and simultaneous injection-molding and decoration in which a decorative sheet inserted in a mold is integrated with a melted resin injected in a cavity during injection molding to decorate the surface of the resin compact (for example, Patent documents 2 and 3).
The above-mentioned decorative resin-molded article is provided with a surface protective layer in order to improve the scratch resistance on the surface. However, the above-mentioned method of forming such a decorative resin-molded article has a problem in the process of three-dimensionally forming a decorative sheet with a vacuum forming mold in insert molding and in the process of drawing and firmly attaching a decorative sheet along the inner periphery of a cavity during preforming or during injecting a melted resin in simultaneous injection-molding and decoration. This problem causes the decorative sheet to be drawn more than minimum requirement to fit the shape of the mold due to the effect of vacuum or compressed air or due to the tension generated by the pressure and the shear stress of the melted resin, resulting in a crack generated on the surface protective layer of the curved surface of a molded article.
To approach the above-mentioned problem, an ionizing radiation curable resin such as an ultraviolet curable resin has been used as the surface protective layer for improving the crosslink density of the resin forming the surface protective layer of a decorative sheet so as to attempt to improve the abrasion resistance and the scratch resistance of the surface of a decorative resin-molded article. However, the problem of a crack generated on the curved surface of a molded article during forming still exists.
Alternatively, an ionizing radiation curable resin such as an ultraviolet curable resin used as the surface protective layer has been attempted to be half-cured at the stage of a decorative sheet and then fully cured after decorative molding (see Patent document 4). However, problems are created, in which the surface protective layer containing an uncured resin component is easily scratched and hardly handled and in which the mold is contaminated due to the uncured resin component adhering to the mold. To solve these problems, a protection film may be provided on a half-cured surface protective layer, but this method complicates the manufacturing process and causes cost increases.
Therefore, the surface protective layer with scratch resistance as well as three-dimensional formability is desired.
A resin composition containing a polycarbonate (meth)acrylate is known (for example, Patent documents 5-10), and a resin composition containing a small amount of yellowing polycarbonate urethane acrylate oligomer is used for the inner colored sheet on the back side of the surface transparent sheet of a decorative sheet for insert molding (Patent document 11). However, no polycarbonate(meth)acrylates are used for the surface protective layer of a decorative sheet.
An acrylic silicon resin has the structure in which the acrylic polymer chain is strongly cross-linked by a siloxane bond, providing excellent weatherability, heat resistance, chemical resistance, and water resistance so as to be widely used in exterior paint. However, if an acrylic silicon resin is used as the surface protective layer in order to improve the scratch resistance on the surface of a resin-molded article, the formed film becomes hard and fragile, possibly causing a crack. To prevent a crack from being generated, a curing process such as ultraviolet curing is applied to a sheet for insert molding after vacuum forming or to a resin molded article after injection molding when an acrylic silicon resin is used as the surface protective layer (for example, see Patent documents 12-14).
However, the curing process applied to a three-dimensionally molded article is complicated with poor economical efficiency, hardly providing uniform curing.
Therefore, the surface protective layer, which has three-dimensional formability as well as scratch resistance while maintaining the excellent chemical resistance of the acrylic silicon resin, is desired